X-ray imaging is used, for example, in CT imaging. Modulation of the radiated X-rays is provided, for example, by modulation of the electron beam, such as with deflection means, or also by providing a varying electrical energy for the generation of the electron beam. US 2010/0020938 A1 describes an anode disk with marks capable of modulating the number of stray electrons detected by a detecting unit. A pattern of marks is provided beside the desired track of the focal spot so that a corresponding pattern in the signal only occurs when the focal spot deviates from the desired track. Thus, it can be detected if the focal spot position leaves the optimum path. However, when applying X-ray radiation in CT imaging, for example it would be helpful for immediate calibration purposes of signal integrating detectors, to modulate the X-ray flux, which is emitted by the source, with at least one or more modulation periods per X-ray frame, thus, within ca. 200 microseconds. US2010/0172475 A1 describes means for dose modulation by deflection of an electron beam into a beam dump. However, the above mentioned examples for modulation of an X-ray beam do not provide a sufficiently fast periodic modulation, while the imaging capabilities of the system like e.g. the focal spot position—are fully maintained.